WO 00/02869, incorporated herein by reference as background art, discloses polyphosphate salts of 1,3,5-triazine compounds which have an average condensation coefficient (number average) of greater than 20 and a molar ratio of triazine compound such as melamine to phosphorus (M/P)>1.1. The WO-specification also describes a two-stage process for the production of those salts by conversion of a 1,3,5-triazine compound with orthophosphoric acid into the corresponding orthophosphate salt as well as thermal treatment for conversion of the orthophosphate salt into a polyphosphate of the 1,3,5-triazine compound. In addition to the orthophosphates it is also possible to use pyrophosphates. The polyphosphate salts described in that publication are desirably to be used as flame-retardant agents.
WO-specification WO 97/44377 describes a melamine polymetaphosphate of a solubility of 0.01 to 0.10 g/100 ml in water at 25° C., a pH-value of 2.5 to 4.5, in the form of a 10% aqueous slurry at 25° C. and a melamine content of 1.0 to 1.1 mole per mole of phosphorus. That melamine polymetaphosphate is also obtained in a two-stage process in which in a first stage melamine, urea and an aqueous orthophosphoric acid solution are mixed together in such a ratio that the molar ratio of melamine to orthophosphoric acid is 1.0 to 1.5 and the molar ratio of urea to orthophosphoric acid is 0.1 to 1.5. The reaction is effected at a temperature of 0 to 140° C. with the removal of water, a powder double salt of orthophosphoric acid, melamine and urea being obtained. In a second stage that is calcined at a temperature of 240 to 340° C. and in that case affords melamine polymetaphosphates. That publication also refers to flame-retardant agents as the field of use for the melamine polymetaphosphates.
WO 00/02869 refers to the disadvantage of the melamine polymetaphosphates in accordance with WO 97/44377 that the melamine polymetaphosphates produced in accordance with the process described therein are unsuitable for the use as flame-retardant agents in polymers, in particular in polyamides and polyesters which are typically processed at elevated temperatures. According to the information set out in WO 00/02869 those melamine polymetaphosphates have inadequate heat resistance, impact strength, tensile strength and breaking strength. There was therefore no reason for the man skilled in the art to find suggestion in WO 97/44377 for example for improving the polyphosphate salts in accordance with WO 00/02869.
European laid-open application No 1 386 942 discloses flame-retardant agents which include a phosphinate or diphosphinate together with a 1,3,5-triazine compound, which are intended to improve the effect over the individual substances.
When using polyphosphate salts in accordance with WO 00/02869 as flame-retardant agents in plastic materials, in particular in glass fibre-reinforced polyamides, polyesters, such as polyethylene terephthalate and polybutylene terephthalate which are usually processed at relatively high temperatures (more specifically above 320° C.), premature partial decomposition was also noted, which on the one hand lead to an attack by the decomposition products on the moulding tools used and which later when used for example as electrical insulating lacquers on copper wires lead to an attack on the latter.